Novel characteristics of cassava, Manihot esculenta Crantz, a reputed C3-C4 intermediate photosynthesis species

Abstract

The cassava plant, Manihot esculenta, grows exceptionally well in low fertility and drought prone environments, but the mechanisms that allow this growth are unknown. Earlier, and sometimes contradictory, work speculated about the presence of a C4-type photosynthesis in cassava leaves. In the present work we found no evidence for a C4 metabolism in mature attached cassava leaves as indicated i) by the low, 2 to 8%, incorporation of (14)CO2 into C4 organic acids in short time periods, 10 s, and the lack of (14)C transfer from C4 acids to other compounds in (12)CO2, ii) by the lack of C4 enzyme activity changes during leaf development and the inability to detect C4 acid decarboxylases, and iii) by leaf CO2 compensation values between 49 and 65 μl of CO2 1(-1) and by other infrared gas exchange photosynthetic measurements. It is concluded that the leaf biochemistry of cassava follows the C3 pathway of photosynthesis with no indication of a C3-C4 mechanism.However, cassava leaves exhibit several novel characteristics. Attached leaves have the ability to effectively partition carbon into sucrose with nearly 45% of the label in sucrose in about one min of (14)CO2 photosynthesis, contrasting with 34% in soybean (C3) and 25% in pigweed (C4). Cassava leaves displayed a strong preference for the synthesis of sucrose versus starch. Field grown cassava leaves exhibited high rates of photosynthesis and curvilinear responses to increasing sunlight irradiances with a tendency to saturate only at high irradiances, above 1500 μmol m(-2) s(-1). Morphologically, the cassava leaf has papillose epidermal cells on its lower mesophyll surface that form 'fence-like' arrangements encircling guard cells. It is proposed that the active synthesis of sugars has osmotic functions in the cassava plant and that the papillose epidermal cells function to maintain a healthy leaf water status in various environments.

International research on cassava photosynthesis, productivity, eco-physiology, and responses to environmental stresses in the tropics

“…Aslam et al 1977, Mahon et al 1977a,b, Edwards et al 1990, Angelov et al 1993, Ueno and Agarie 1997 are of a limited value if such results are to be interpreted in relation to the real potential of cassava and to the underlying mechanisms controlling the overall photosynthetic process (El-Sharkawy and Cock 1987a, El-Sharkawy et al 1992a, El-Sharkawy 2004. Lower P N of potted cassava grown in cabinets or greenhouses were probably the result of lower activities of photosynthetic enzymes, as has long been observed in other plant species, and/or changes in leaf anatomy because of exposure to suboptimal PPFD and air temperature during leaf development, plus other consequences such as imbalances in source-sink relations in the whole plant system and possible feed-back inhibition of P N resulting from restricted root sinks for assimilates (particularly when growing cassava in small pots) (see for example Nösberger and Humphries 1965, Humphries 1967, Neales and Incoll 1968, Moss and Musgrave 1971, Nobel 1976, Boardman 1977, Herold 1980, Nobel and Hartsock 1981, Šesták 1985, Bunce 1986, Ho 1988, Evans 1993, Wardlaw 1990, Pellet and ElSharkawy 1994.…”

Salt and Water Deficit Tolerance in some Vicia faba L. Genotypes in Relation to Pigments, ISSR – PCR Markers and Stress Tolerance Indices

“…Leaf sugar was extracted according to the method described by Angelov et al (1993) and Total soluble sugar (TSS) were determined using the methods of Riazi et al (1985). The starch content was estimated by the method prescribed by Hedge and Hofreiter (1962).…”

Improved Growth And Colchicine Concentration In <i>Gloriosa Superba</i> On Mycorrhizal Inoculation Supplemented With Phosphorus-Fertilizer

“…Later, the same dried shoot samples were analyzed for their mineral concentration. Leaf sugar was extracted by the method of Angelov et al (1993). Total soluble sugar was determined using the method of Riazi et al (1985).…”